Pulsed arc welding was first proposed in the early 1960's and since that time has become one of the most useful developments in arc welding. However, pulsed arc welding has never reached its full potential due mainly to the number and complexity of the controls which an operator must manipulate in order to achieve satisfactory welding conditions, in particular, the most desirable welding condition whereby one welding metal droplet is detached with each welding current pulse irrespective of the wire feed rate.
The variables which exist in a pulsed arc welding system include:
(a) pulse height--the amplitude of the current pulse, PA1 (b) pulse width--the duration of the current pulse, PA1 (c) wire feed rate--the lineal speed at which the electrode is fed to the welding arc, PA1 (d) pulse frequency--the repetition rate of the current pulses, PA1 (e) background current--the relatively low value of DC current which flows in the arc in the periods between current pulses, PA1 (f) the arc current, and PA1 (g) the arc voltage. PA1 (1) as measured during the pulse period only, PA1 (2) as an average of the pulse and background arc voltage, or PA1 (3) as measured in the background period only. PA1 (a) the voltage drop across the electrode wire extension, PA1 (b) the voltage drop across the contact tip/electrode wire interface, and/or PA1 (c) the voltage drop across the connecting leads between the welding power source and the welding arc.
The pulse height in combination with the pulse width define the energy content of the pulse and this must always be sufficient to form and detach the weld metal droplet. The pulse height must be above "the threshold" current level in a particular wire size/type/shielding gas combination necessary to achieve spray transfer of the weld metal. It is desirable for a practical welding system to include a feedback mechanism of some type to take account of changes in the welding parameters, such as variations in mains voltage and wire feed rate due to changes in the motor speed or slippage in the wire feed system. Although various forms of feedback system have been proposed, none have had the desired effect of maintaining welding parameters which ensure the detachment of a single droplet for each current pulse generated by the system. This is principally due to the fact that the circuitry does not adequately adapt to the changes in welding conditions caused by changes in the wire feed rate and changes in the position of the electrode relative to the workpiece (arc length). Most attempts to compensate for such changes have concentrated on controlling the pulse width to maintain a relatively constant electrode burn-off rate. However, such an approach ignores the importance of the correct relationship between the pulse frequency and the wire feed rate resulting in the production of more than one metal droplet per current pulse, or in the need for more than one pulse for the detachment of the metal droplet, which in turn results in spatter of the weld metal and other undesirable effects.
Various attempts have been made to overcome the above described problems, and while some have met with substantial success, a number of important shortcomings still remain. For example, U.S. Pat. Nos. 4,409,465 Yamamoto et al and 4,438,217 Ueguri et al discloses essentially the same approach to the control of arc current pulses in which the voltage across the arc is monitored and a feedback circuit is used to maintain the arc voltage at a predetermined desired average level. As indicated in the above patents, the voltage across the arc may be measured in one of the following ways:
In the first measurement method listed above, since the measurement is taken at the time of maximum current, any practical voltage measurement is likely to include significant voltage drop components due to:
Such voltage drops cannot readily be compensated for, especially where the electrode wire extension changes according to operator technique.
Where the arc voltage is measured in accordance with item (2) above, since there is a ratio of approximately 2:1 between the pulse and background voltage levels, especially at low wire-feed rates where the pulse/background duty cycle is low, an excessive averaging time must be applied to the arc voltage signal. This of course adds undesirable lag in the control system. Where the wire-feed rate is high, the pulse/background duty cycle is correspondingly high and the effects detailed for voltage measurement during the pulse period only necessarily prevail.
Where the arc voltage is measured during the background period only, the voltage drop problems detailed above are less significant since background current levels are typically much lower than pulse current levels. However, if the arc voltage level during the background period is averaged, the accuracy of the result can be compromised by the occurrence of short circuiting within the arc. This typically occurs immediately following a pulse period, and especially where the machine has been set to produce a short arc length. It will be appreciated that when the arc length is short, a bridge of molten electrode metal may still be intact between the electrode wire and the detaching droplet at the instant at which the droplet contacts the workpiece or molten weld pool. This short circuiting naturally affects the average arc voltage and results in inaccuracy in the control applied to the arc welding system.
The above described problems are especially apparent for high resistivity electrodes, such as Inconel, and it is one object of the present invention to provide a pulse arc welding system which at least reduces the above problems and improves control over the welding process.